ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
517 
while it may occasionally have been an indirect, has rarely been the 
direct cause of the extinction of species. It is equally doubtful if 
extinction has been due as a general rule to lack of suitable food. Most 
probably the usual causes of destruction have been adverse conditions of 
nature and the competition of other species in the struggle for food. 
Though bacteria and other disease-producing agents may at times in the 
past have attacked species of animals destructively, it is probable that 
they have played a minor part in the extinction of species. One tendency, 
which has particularly manifested itself in herbivorous forms and has 
frequently led to their destruction, is the tendency to increase in size 
through the double influence of abundance of food and little waste of 
tissue through exercise. The great Cretaceous reptiles may to some extent 
have been affected by this, but they could hardly have succumbed to it. 
What reptiles want, however, is the instinct of carefulness for their eggs, 
which is not much more developed in them than it is in fishes. Fish 
species indeed escape annihilation mainly through fecundity, but the 
huge Cretaceous reptiles probably laid but few eggs, and these they had 
to lay on land, where they were in all probability devoured by the small 
mammals which had then begun to appear. It is highly probable that 
the placental mammals who were slowly developing intelligence adopted 
from time to time new methods of attack, while the reptiles were very 
much slower in developing new methods of defence. The egg-destroying 
mammals had a double advantage ; laying no eggs themselves and caring 
for their young they could only be destroyed when in the mature stage, 
while their assault upon their foes was by the safer and more effective 
process of devouring them in the egg — a method which may well have 
caused rapid reduction in numbers and final extinction. With regard 
to the more recent large mammals, while it is probable that the Glacial 
epoch may have had something to do with their disappearance, this is 
not an explanation which can be applied to the South American Horse, 
or to its huge contemporaries, the Megatherium and the like. It is 
suggested that some active carnivorous animal began to attack and 
destroy the young of the Giant Sloths in a new and covert manner 
which the parents were quite incapable of guarding against. With 
regard to the South American Horse no explanation can be suggested 
which appears to be satisfactory. 
Study of Metamerism.* — Prof. T. H. Morgan discusses in a series 
of chapters the phenomena of metamerism. He deals first with typical 
forms of modification in Annelids ; the variation in the position of the 
reproductive organs is next considered ; thirdly, limiting himself to 
the anterior part of the body, he discusses the abnormalities which may 
be observed in it. These variations are, it is suggested, due to a 
regeneration in the adult of anterior metameres. A study of embryos 
showed that all the commoner forms of abnormalities recorded for adult 
forms are found also in them. Abnormalities at the posterior end are 
frequently found and appear to be due to the conditions acting during 
regeneration, and not to be in any way connected with an hereditary 
tendency to be more abnormal in one case than in another. That is to 
say, the tissues of a worm that has developed normally from the egg are 
* Quart. Journ. Micr. Sci., xxxvii. (1895) pp. 395-476 (4 pis.). 
2 M 
1895 
